1. Field of the Invention
This invention relates to a cathode for use in an electrolytic process, and more particularly to an improved cathode structure for use in an electrolytic process in which the anode and cathode are separated by an ion-permeable membrane. The cathode structure is especially well-adapted for use in an apparatus for the production of galvanized metal sheet or strip material having a zinc coating on one side only.
2. Description of the Prior Art
Electrochemical or electrolytic processes, and apparatus for their performance, are known in which anode and cathode means are immersed in anolyte and catholyte solutions, respectively, with the solutions being confined to contiguous chambers separated by an ion-permeable wall or membrane. It is also known to flow the anolyte and catholyte solutions through their separate chambers, one such apparatus being shown, for example, in U.S. Pat. No. 3,945,892, in which a bed of anode particles is fluidized by the upward flow of anolyte solution through the anode chamber. The cathode compartment is separated from the particulate anode by a semipermeable membrane, and means are provided for increasing the pressure in the cathode compartment to equalize the increased hydrostatic pressure required to produce the flow through the particulate anode and to thereby avoid rupturing the membrane.
A method of producing galvanized metal or sheet material having a zinc coating on one side only is disclosed in U.S. Pat. No. 3,988,216, which is assigned to the assignee of this invention. According to this patent, a strip of metal which has been previously coated on both sides by hot dipping or other suitable process is drawn through an electrolyte bath between an anode immersed in the bath and a cathode immersed in an electrolyte solution which is kept separated from the anode bath solution by a perm-selective anion membrane. By applying negative current to the cathode and positive current to the anode, zinc is removed from the side of the strip facing the cathode and a substantially equal amount of zinc is simultaneously plated onto the side facing the anode.
In the production of galvanized sheet or strip steel, including one-side galvanized strip produced according to the method of U.S. Pat. No. 3,988,216, relatively high current densities are required in order to plate the strip as it is drawn through the apparatus at a commercially acceptable rate. Further, the strip may be up to 6 feet or more in width, with the anode and cathode having surface areas facing the strip of up to 50 square feet or more. With current densities of up to 1000 amperes per square foot (asf) employed, a substantial amount of heat is generated which heats the electrolyte solutions in which the anode and cathode are immersed. Since the anion membranes conventionally used in such devices are formed from a thin, delicate sheet material such as a thermoplastic synthetic resin material, the heat in the catholyte solution surrounding the cathode presents serious problems in utilizing the method on a commercial scale for producing galvanized sheet steel.
In the process of producing one-side galvanized strip steel in which the anode is immersed in an acidic electrolyte solution, or anolyte, and the cathode in a basic electrolyte solution, or catholyte, zinc coating on the side of the strip adjacent the cathode is oxidized to zinc ions which go into solution, while a substantially equivalent amount of zinc ions are reduced to zinc metal and deposited from the solution on the side of the strip facing the anode. Water disassociates at the anode and the cathode, with hydroxyl ions and hydrogen gas being generated at the cathode and hydrogen ions and oxygen gas being generated at the anode. The hydroxyl ions carry the electrocurrent through the perm-selective anion membrane and reunite with the hydrogen ions to re-form water. However, the hydrogen gas generated at the surface of the cathode tends to interfere with the electrolytic action when permitted to accumulate and form bubbles on the surface of the cathode.
In view of the foregoing, it is a primary object of the present invention to provide an improved cathode structure for use in an electrolytic process.
It is a further object of the invention to provide an improved cathode structure for use in an electrolytic process in which the anode and cathode are immersed in separate electrolytic solutions separated by an ion-permeable membrane.
Another object of the invention is to provide an improved cathode structure for use in the production of one-side galvanized sheet or strip metal material.
Another object of the invention is to provide such a cathode structure having means for circulating an electrolyte solution over the surface of the cathode at a rate sufficient to effectively cool the cathode and to flush hydrogen gas from the cathode surface.